空指针异常:无法解析 JSON 数据.(空对象引用)

Question

我正在尝试从url获取数据。我已经阅读了很多关于同一问题的帖子，但没有任何效果。请帮我解决这个问题...在下面发布我的代码

Json解析器.class

import android.util.Log;
public class JsonParser {
    final String TAG = "JsonParser.java";
    static InputStream is = null;
    static JSONObject jObj = null;
    static String json = "";
    public JSONObject getJSONFromUrl(String url) {
        // make HTTP request
        try {
            DefaultHttpClient httpClient = new DefaultHttpClient();
            //HttpPost httpPost = new HttpPost(url);
            HttpGet httpGet = new HttpGet(url);
            HttpResponse httpResponse = httpClient.execute(httpGet);
            HttpEntity httpEntity = httpResponse.getEntity();
             String json = EntityUtils.toString(httpResponse.getEntity());
            is = httpEntity.getContent();           
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        } catch (ClientProtocolException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        try {
            BufferedReader reader = new BufferedReader(new InputStreamReader(is, "iso-8859-1"), 8);
            StringBuilder sb = new StringBuilder();
            String line = null;
            while ((line = reader.readLine()) != null) {
                sb.append(line + "n");
            }
            is.close();
            json = sb.toString();
        } catch (Exception e) {
            Log.e(TAG, "Error converting result " + e.toString());
        }
        // try parse the string to a JSON object
        try {
            jObj = new JSONObject(json);
        } catch (JSONException e) {
            Log.e(TAG, "Error parsing data " + e.toString());
        }
        // return JSON String
        return jObj;
    }
}

这是我的主要活动

public class MainActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        // we will using AsyncTask during parsing 
        new AsyncTaskParseJson().execute();
    }
    // you can make this class as another java file so it will be separated from your main activity.
    public class AsyncTaskParseJson extends AsyncTask<String, String, String> {
        final String TAG = "AsyncTaskParseJson.java";
        // set your json string url here
        String yourJsonStringUrl = "http://localhost:8083/android/search";
        // contacts JSONArray
        JSONArray dataJsonArr = null;
        @Override
        protected void onPreExecute() {}
        @SuppressLint("NewApi")
        @Override
        protected String doInBackground(String... arg0) {
            try {
                // instantiate our json parser
                JsonParser jParser = new JsonParser();
                // get json string from url
                JSONObject json = jParser.getJSONFromUrl(yourJsonStringUrl);
                // get the array of users
                dataJsonArr = new JSONArray(json.getJSONArray("searchlist"));
                // loop through all users
                for (int i = 0; i < dataJsonArr.length(); i++) {
                    //JSONObject c = dataJsonArr.getJSONObject(i);
                    //String firstname = c.getString("firstname");
                    String firstname= dataJsonArr.getString(i);
                    // show the values in our logcat
                    Log.e(TAG, "firstname: " + firstname 
                            );
                }
            } catch (JSONException e) {
                e.printStackTrace();
            }
            return null;
        }
        @Override
        protected void onPostExecute(String strFromDoInBg) {}
    }
}

安德罗伊清单.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.androidjsonparsing"
    android:versionCode="1"
    android:versionName="1.0" >
    <uses-permission android:name="android.permission.INTERNET"></uses-permission>
    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="17" />
    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="com.example.androidjsonparsing.MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>
</manifest>

这是我们从 url 获取的 json 文件：

  {"searchlist":["Micromax Ninja A89 Smartphone","Capdase Fliptop Case for Samsung Galaxy S II I9100","IFB Microwave Convection 20SC2, 20 litres","Samsung Microwave Convection CE1041DFB, Black, 28 litres"]}

这是我的日志文件：

04-23 13:34:49.621: E/JsonParser.java(955): Error converting result java.lang.NullPointerException: lock == null
04-23 13:34:49.622: E/JsonParser.java(955): Error parsing data org.json.JSONException: End of input at character 0 of 
04-23 13:34:49.622: E/JsonParser.java(955): --------- beginning of crash
04-23 13:34:49.662: E/AndroidRuntime(955): FATAL EXCEPTION: AsyncTask #1
04-23 13:34:49.662: E/AndroidRuntime(955): Process: com.example.androidjsonparsing, PID: 955
04-23 13:34:49.662: E/AndroidRuntime(955): java.lang.RuntimeException: An error occured while executing doInBackground()
04-23 13:34:49.662: E/AndroidRuntime(955):  at android.os.AsyncTask$3.done(AsyncTask.java:304)
04-23 13:34:49.662: E/AndroidRuntime(955):  at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:355)
04-23 13:34:49.662: E/AndroidRuntime(955):  at java.util.concurrent.FutureTask.setException(FutureTask.java:222)
04-23 13:34:49.662: E/AndroidRuntime(955):  at java.util.concurrent.FutureTask.run(FutureTask.java:242)
04-23 13:34:49.662: E/AndroidRuntime(955):  at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
04-23 13:34:49.662: E/AndroidRuntime(955):  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
04-23 13:34:49.662: E/AndroidRuntime(955):  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
04-23 13:34:49.662: E/AndroidRuntime(955):  at java.lang.Thread.run(Thread.java:818)
04-23 13:34:49.662: E/AndroidRuntime(955): Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'org.json.JSONArray org.json.JSONObject.getJSONArray(java.lang.String)' on a null object reference
04-23 13:34:49.662: E/AndroidRuntime(955):  at com.example.androidjsonparsing.MainActivity$AsyncTaskParseJson.doInBackground(MainActivity.java:50)
04-23 13:34:49.662: E/AndroidRuntime(955):  at com.example.androidjsonparsing.MainActivity$AsyncTaskParseJson.doInBackground(MainActivity.java:1)
04-23 13:34:49.662: E/AndroidRuntime(955):  at android.os.AsyncTask$2.call(AsyncTask.java:292)
04-23 13:34:49.662: E/AndroidRuntime(955):  at java.util.concurrent.FutureTask.run(FutureTask.java:237)
04-23 13:34:49.662: E/AndroidRuntime(955):  ... 4 more

Answer 1

In MainActivity

更改此行：

dataJsonArr = new JSONArray(json.getJSONArray("searchlist"));

自：

dataJsonArr = new json.getJSONArray("searchlist");

否则编码就是工作。

检查您提供的网址是否正确。